摘要：

Several samples of thymus desoxyribose nucleic acid in saline solutions have been studied by light scattering, viscosity and sedimentation methods after exposure to a series of elevated temperatures. For exposure times of one hour, changes are noticed at temperatures above 89° for two of the samples and above somewhat lower temperatures for the others. From this characteristic temperature up to 100° the viscosity falls about 12-fold (as Zamenhof first noted), the mean end-to-end length decreases by about 15-fold and the molecular weight remains unchanged according to light scattering studies. Sedimentation measurements reveal a broadening of the distribution of sedimentation constants but no significant change in the average value. Typical denaturing agents lower the temperature range in which these changes occur by 10-20° but have no effect at room temperature. It appears that the changes at the elevated temperatures are due to the melting out of regions of hydrogen-bonded structure within DNA molecules and this cooperative breakdown of the highly organized, native structure is termed denaturation. At a given elevated temperature the viscosity quickly reaches an equilibrium value but it cannot be decided whether this indicates existence of a true equilibrium between native and denatured regions of the DNA molecules or a distribution of denaturation temperatures for different regions. It is shown that the completely denatured state can be achieved without chain degradation and it is surmised that in this state the polynucleotide strands are held together by remaining or reformed hydrogen bonds.

展开